Vertical or angled shafts in mining are commonly produced by drilling a relatively small diameter pilot hole downwardly into the earth with a pilot bit. Once the desired depth of the hole is reached, the pilot bit is removed and replaced by a much larger diameter raise drill bit. The raise drill bit is rotated and simultaneously pulled upwardly along the pilot hole by a drill string extending downwardly through the pilot hole from a drilling rig located at an upper elevation.
Typically, a raise bit includes a cutter carrier frame and a drive stem extending upwardly from the cutter carrier frame for detachable connection with the drill string extending downwardly through the pilot hole. A plurality of roller cutters are mounted on the upper surface of the cutter carrier frame to disintegrate the earth formations surrounding the pilot hole. The roller cutters commonly have peripheral cutting edge portions which protrude into the face of the raise hole to cut concentric kerfs upon rotation of the raise bit. During reaming operations, the drive stem is subjected to a tremendous upward pull and to an extremely high torque load to rotate the raise bit. The rotational movement of the raise bit is typically erratic with the raise bit continuously increasing and slowing down in rotational speed. This erratic movement imposes cyclical torque loads on the drive stem which tends to fatigue the stem. Also, although the drive stem is guided by the pilot hole, the erratic manner in which the rock at the face of the raise hole fractures tends to cause the raise bit to rock or tilt as it advances upwardly, thereby loading the drill stem in bending. Ultimately, the combined effect of all of these high level loads commonly results in the failure of the drive stem.
Accordingly it is important to design the drive stem to carry as large a load as possible. However, the maximum load which a drive stem is capable of carrying is often significantly reduced by modifications made to the stem to accommodate the center or innermost roller cutters which are used to disintegrate the portions of the earth formation immediately surrounding the pilot hole. The innermost cutters must be located close enough to the pilot hole to ensure that all of the rock surrounding the pilot hole is excavated. If the innermost cutters are positioned too far away from the pilot hole, an annular core may be left surrounding the drive stem. To prevent this from occurring, several alternatives have been used to place the central or inboard cutters as close as possible to the drive stem. In U.S. Pat. Nos. 3,675,729 and 4,108,259, a transverse blind bores or holes are formed in the drive stems for receiving the radially inwardly disposed end of a shaft used to rotatably support a center roller cutter. Such blind holes not only reduce the effective cross-sectional area of the drive stem, but also create stress risers in the drive stems, thereby significantly reducing their capacity to carry the tensile, torsional and bending loads imposed thereon during typical raise drilling operations.
Another commonly employed manner of positioning the center cutters as close to the pilot hole as possible is to relieve or notch the drive stem to provide clearance for the adjacent portion of the saddle used to mount the roller cutters on the cutter carrier frame. Examples of raise bits using this particular alternative are disclosed by U.S. Pat. Nos. 3,917,009, 4,010,808, 4,042,047 and 4,177,866. An obvious drawback of the drive stems disclosed in these patents is that the notch or relief reduces the effective cross-sectional area of the drive stem. Moreover, the relief or notch may create a stress riser in the stem.
In U.S. Pat. No. 4,004,644, the radially inwardly directed end portion of the central roller cutter saddles are welded directly to the drive stem. While this particular technique may locate the central cutters fairly close to the pilot hole, the weldment induces significant stress concentrations on the drive stem thereby reducing its load carrying capability.